SECT. XVIII. 3. OF SLEEP. 155 



of imagination for ideas excited by irritation ; as perpetually 

 happens to people debilitated by fevers on their firft awaking : 

 for in thefe fevers with debility the general quantity of irrita- 

 tion being diminifhed, that of fenfation is increafed. In like 

 manner if the aftions of the flomach, inteftines, and various 

 glands, which are perhaps in part at lead caufed by or catenated 

 with agreeable fenfation, and which perpetually exift during 

 our waking hours, were like the voluntary motions fufpended in 

 our fleep ; the great accumulation of fenforial power, which 

 would necefiarily follow, would be liable to excite inflammation 

 in them. 



3. When by our continued pofture in fleep fome uneafy 

 fenfations are produced, we either gradually awake by the exer- 

 tion of volition, or the mufcles connected by habit with fuch 

 fenfations alter the pofition of the body ; but where the fleep is 

 uncommonly profound, and thofe uneafy fenfations great, the 

 difeafe called the incubus, or nightmare, is produced. Here the 

 defire of moving the body is painfully exerted, but the power cf 

 moving it, or volition, is incapable of a&ion, till we awake. 

 Many Tefs difagreeable druggies in our dreams, as when we wifh 

 in vain to fly from terrifying objecls, conftitttte a flighter degree 

 of this difeafe. In awaking from the nightmare I have more 

 than once oblerved, that there was no diforder in my pulfe ; 

 nor do I believe the refpiration is laborious, as fome have affirm- 

 ed. It occurs to people whofe fleep is too profound, and fome 

 difagreeable fenfation exifts, which at other times would have 

 Awakened them, and have thence prevented the difeafe of night- 

 mare ; as after great fatigue or hunger with too large a fupper and 

 wine, which occafion our fleep to be uncommonly profoun^i. 

 See No. 14, of this Section. 



4. As the larger mufcles of the body are much more fre- 

 quently excited by volition than by fenfation, they are but fel- 

 dom brought into aHon in our fleep : but the ideas of the mind 

 are by habit much more frequently connected with fenfation 

 than with volition ; and hence the ceafelefs flow of our ideas in 

 dreams. Every one's experience will teach him this truth, for 

 we all daily exert much voluntary mufcular motion : but few 

 of mankind can bear the fatigue of much voluntary thinking. 



5. A very curious circumftance attending thefe our fleeping 

 imaginations is, that we feem to receive them by the fenfes. 

 The mufcles, which are fubfervient to the external organs of 

 fenfe, are connecled with volition, and ceafe to al in fleep ; 

 hence the eyelids are clofed,and the tympanum of the ear relaxed ; 

 and it is probable a fimiiarity of voluntary exertion may be necef- 

 fary for the perceptions of the other nerves of fenfe ; for it it 



obferved 



